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The proposed variant of the capacitive proximity sensor is economical, works in a wide interval of values of the supply voltage and has a high stability threshold when the temperature changes.

Knigakh and magazines to radio Amateurs for the last 20 years published many descriptions of structures of proximity sensors, which differ in principle of operation, sensitivity, complexity, and used components. However, many of them suitable for use only in conditions close to the laboratory, with almost constant ambient temperature and supply voltage.

For example, the sensor described in [1], performed on a digital chip and has high efficiency, however, the threshold of response depends significantly on supply voltage. Stability under high humidity due to high resistance of the resistor R2 is clearly insufficient and strongly depends on the length the wires connecting the electronic unit with the sensor element.

The sensors proposed in [2], consume a current of several milliamps, limits the possibility of their application in systems with Autonomous power supply. Because the dependence of the threshold characteristics of the shelter from temperature and supply voltage possible the situation when such a sensor will be triggered condition, or cease to operate altogether.

The proposed sensor is a little harder mentioned above, but differs from them the lack of winding elements, good repeatability, operates at power supply voltage 3...15 V while consuming approximately 40 μa at a voltage of 5 In). It is characterized by independence of the threshold temperature environmental and supply voltage, low sensitivity to electromagnetic interference and crosstalk. Possible the exact calculation of the threshold on the basis of of the values of the elements used, or the calculation of these values to obtain the required threshold.

Sensor circuitry shown in Fig. 1. On the trigger DD1.1 is a generator pulses. Their duration (about 0.2 MS) set circuit R1C1, and the period repetition (approximately 1.5 MS) - circuit R2C2. The detector low voltage DA1 some time after power-on of the device keeps the voltage at the input S of the trigger DD1.1 at a low logic level, thereby eliminating prohibited high-level status on both the installation inputs (R and S) trigger. Otherwise, in case of rise of voltage at a speed of less than 2...3 In/MS-excitation of the generator will occur.

The pulse generator simultaneously launch two single vibrator. The first (on the trigger DD2.1) generates pulses exemplary duration dependent of the values of elements R4, R5, C4. The pulse duration of the second single vibrator (on the trigger DD2.2) depends on the resistance of the resistor R3 and the capacitance of the capacitor, formed metal plates E1 and E2. The coupling capacitor C5 prevents accidental hit to the input of the flip-flop DD2.2 DC voltage.

Sensor operation is based on comparing the pulse duration generated by two the single vibrators. If the momentum of the second (measuring) the shorter single vibrator pulse first (exemplary), at the time of the positive voltage drop of the inverted output of flip-flop DD2.1 (point 1, Fig. 1) the voltage level on the output of the flip-flop DD2.2 (point 2) will be low. Trigger comparison DD1.2, trigger on positive slope at the input, goes into a state of low logic level at the output. Otherwise (measuring pulse longer than exemplary) level at point 2 and the output of the trigger DD1.2 will be high.

When approaching a foreign object to the plates E1 and E2 capacitance between the two increases, the low level at pin 2 of connector X1 is replaced high. The threshold capacity beyond which it occurs, determine according to the formula

where R4BB - entered the resistance of the tuning resistor R4; TSW ≈ 6 PF - the capacitance of the input R of the trigger. When specified in the scheme the value of the resistor R5 through R4, you can change the threshold in capacity from 6 to 32 pF.

Since the active elements of the multivibrators are inside one chip DD2, when the temperature or voltage characteristics and the duration of generated pulses are changed equally. This provides the stability threshold of the sensor in a wide range of changes temperature and supply voltage.

In the sensor, you can use fixed resistors C2-SN, MLT, S2-23 or similar power 0,125 or 0,25 W tolerance ±5 %. As R4 it is desirable to use a trimmer with a small TCS (e.g., GPA-19a, SDR-196). Widespread resistors SDR-38A for this reason does not apply recommended. Capacitors C1 - C4 - any miniature ceramic (km-5, Km-6, K10-17 or similar import). The coupling capacitor C5 should to be a high voltage (e.g., K15-5), designed for a voltage of less than 500 V. Its capacity may be in the range from 1000 to 4700 pF. Diode VD1 - any of the series CD, CD, KD521, KD522.

Chip KTM can be replaced by TM or their imported counterparts. Detector low voltage (DA1) should be chosen threshold voltage, obviously a lower minimum supply voltage of the sensor. For example, when powered rated 5 volts suitable detectors CROP, CROP, with 9 In - also CROP, CROP, CROP.

Electronic sensor unit is assembled on the Board of foiled fiberglass 1.5 mm thick. Drawing printed conductors and arrangement of parts shown in Fig. 2. Sensitive elements (plates E1 and E2), it is recommended to take the form "deployed" capacitor [2], combining it with electronic control unit wires length not more than 50 mm.

The establishment of the sensor is reduced to the threshold setting resistors R4 and R5. Actuation can be controlled using the circuit of the led (anode to the pin 2 of connector X1) and resistor 4.7 Ohm 2,2...(between the cathode led and pin 3 of the connector). Turn the power on, rotate the engine trim resistor R4 until the ignition of the led and then turn engine slightly to the right (under the scheme) - its extinction. The correct adjustment will be be indicated by the led on when approaching sensitive the element of any object. If the led is not lit even in the left the position of the slider of resistor R4 should be set instead of the jumper and R5 repeat setting procedure.

The device can be used as a touch sensor of a man to the plate E2, moreover, its role can perform any metal object such as a door handle. In this case, from the plate E1 you may refuse, and the resistors R4 and R5 be replaced by a single resistor of 330 ohms.

One embodiment of a sensor made by the author, had a sensitive element in the form of a flat capacitor with an area of 100 cm2 plates and distance between them 5 mm. It surely worked when filling the space between the plates lubricating oil by 70% in the temperature interval -30. ..+85 °C Actuation, caused by condensation of water, the approach of hands and other interfering factors fixed.

With such use and application as a sensitive element flat or cylindrical capacitor is recommended to evaluate the desired value is entered, the resistance of the tuning resistor R4 by the formula

where SPM is the capacitance of the connecting wires; Ck is the capacitance of the sensing element, calculated from the known formulas capacity flat or cylindrical capacitor.

If the calculated value turned negative, should be excluded from the scheme the resistor R5, and if greater than 200 ohms, to increase the value of R5 so that resistance R4BB lay in the range of 100... 150 kOhm. Finally the sensor regulate the manner described above.

Literature

  • Nechaev I. Capacitive relay. Radio, 1988, No. 1,p. 33.
  • Moskvin A. non-contact capacitive sensors. Radio, 2002, No. 10, pp. 38, 39.
  • Author: M. Ershov, Tula